Process printing is used with different kinds of printing operations, such as lithographic, flexographic, gravure, digital, and other printing processes. In general, the process color set has four colors—black, cyan, magenta, and yellow. Full-color prints are made by color separating the original image into tone values of the process colors. The color separations are printed sequentially to produce the full-color print of the original image.
Sometimes a four-color ink set cannot reproduce the full gamut of colors of the original image. When a truer color reproduction is desired, sometimes one or more “spot colors,” or inks having a specific desired color, are printed in a further step. Another solution has been to use ink sets with more ink colors, for example six- and seven-member ink sets. Examples of such augmented sets are described in Herbert et al., U.S. Pat. No. 5,734,800 (six-color ink set); Bernasconi, U.S. Pat. No. 5,751,326 (seven-color separation process); and Alderliefste, WO 02/071739 (six-color ink set). While using spot colors and bigger ink sets can reproduce an image with more color fidelity, these solutions are more expensive because each additional ink color requires another printing unit on the press. Further, spot color printing units need to be cleaned out when the printing job for which it was employed is completed. Six- and seven-member ink sets are likewise more expensive, because they also require additional printing units on the press.
It would thus be desirable to have a four-color ink set that was capable of reproducing more colors so that a truer image could be obtained without resorting to spot colors or to larger ink sets.